Many different codes are used in data transmission systems, and some require more transmission channel bandwidth than others for the same information transmission rate. One so-called self-clocking code, i.e., a code from which data clock can be recovered, is delay modulation (DM), sometimes called modified frequency modulation (MFM) or Miller coding; and it requires a relatively low bandwidth. It has, therefore, often been used in magnetic recording systems to achieve a relatively high storage packing density. In such use, an associated timing channel is usually provided to assure proper information phase with respect to the system time base because the decoding of DM code is uniquely subject, without close clock tracking, to noise-induced apparent information phase shift that results in ambiguities causing decoding errors.
For example, in an M. F. Davis et al. U.S. Pat. No. 3,864,735, there is shown a coding and decoding arrangement in which a read clock recovery circuit, according to an M. F. Davis et al. U.S. Pat. No. 3,831,195, utilizes a 2-mode phase-locked loop slaved to a clocking track in a magnetic recording medium. The resulting read clock is used in an asymmetrical data window encoder, according to an I. E. Walenta U.S. Pat. No. 3,794,987, to drive monostable multivibrators and tandem flip-flop circuits for converting MFM data to the nonreturn-to-zero (NRZ) format. The relatively narrow bandwidth coding format feature, which facilitates achievement of high density packing in magnetic stores such as that contemplated by Davis et al., is a feature that is also sought in data transmission systems which interface with equipment using other coding formats such as the NRZ format. One such system is an optical data link.
Transmission errors in data transmission systems have been detected by various parity coding schemes and by schemes which do a comparison of multiple transmissions of a single message. These require transmission system facility time and thereby lower the information transmission efficiency.
Data transmission systems also have a need for signaling such as for message synchronization or periodic framing. For example, to initiate a message transmission it is sometimes necessary to employ a time consuming, so-called, handshake procedure in which an alerting signal is sent, a ready signal is returned, and then message transmission begins. On the other hand, systems using a bipolar coding system can transmit polarity violations for certain signaling purposes, but such systems require substantially more channel bandwidth than does delay modulation.